Flex lockup strategies currently employed by auto manufacturers, with intent toward more finely controlled engine RPM and fuel consumption, may provide a marginal improvement. However, this only holds true in new vehicles with system components operating at optimum and under ideal conditions. As systems begin to age the lockup function degrades rapidly resulting the following excess torque converter slip, torque converter clutch “shudder” (torsional vibration generated by slip-grab-slip-grab), excess heat generated in the torque converter, premature failure of the lockup clutch, premature failure of the ATF, and/or, diagnostic trouble codes set (the slip RPM being too high for too long during the flex duty cycle). Many of these symptoms or malfunction are experienced after repair with a remanufactured torque converter (TC) and rebuilt transmission.
With reference to FIG. 1, the diagram illustrates the conventional electronic control of the torque converter clutch (TCC) solenoid during operation, and the corresponding lockup clutch controlled slip RPM during the application cycle. The long gradual apply of the “flex” lockup makes TCC application nearly undetectable by most without a scan device. At lockup off, the slip RPM is 500 and the TCC solenoid is increased from 0 amps to 0.10 to 0.12 amps as the solenoid is being charged in preparation for lockup. At the beginning of the lockup pulsed/controlled slip phase, the solenoid signal is increased to approximately 0.31 amps. During this phase, the slip RPM decreases from 250-280 RPM to 50-80 RPM as the solenoid signal is ramped from 0.31 up to 0.41 amps over approximately 5-7 seconds. At the beginning of the lock up full on phase, the solenoid signal is increased to 0.49 to 0.51 amps and the slip RPM decreases to 0-10 RPM.
With reference to two conventional lockup control systems in FIG. 5, the type 2 pre-fill valve area reciprocal is 22.29 (1/0.04486″). Therefore, with a factory spring tension of 3.375 lb.×22.29=75.23 psi internal TC pressure required to overcome the spring to pre-fill the lockup clutch apply circuit with no solenoid signal applied to the valve. The TC pressure typically runs 10-25 psi below line pressure supply when pressure is below the TC feed limit of 127 psi. Thus, line pressure supply must exceed around 90 psi (25% of the boost range) in order to pre-fill. However, when line pressure specifications are compared and show that the idle line pressure is 54-61 psi, and if line happens to be running on the low side of this specification, it is possible that to have insufficient downward force on the valve to pre-fill the circuit, even with solenoid signal applied at approximately 20% duty cycle during the solenoid charging phase (see FIG. 1) which is intended as the pre-fill cycle. Thus, when lockup is commanded ON, there is a time delay and lower-than-expected TCC apply pressure. This state can occur, e.g., at low RPM cruise with minimal load (relatively low line pressure and low TCC internal psi resulting in reduced pump volume output).